Be very careful
There's something called dry drowning and it happens when a child has swallowed a lot of water either pool or lake etc
Some signs can seem like a stomach bug like nausea and vomiting.
Just FYI

I ALWAYS FREAK OUT WHEN MY DAUGHTER DOES IT. but I constantly tell her to spit it out immediately
Hoping she catches on that it is not allowed,

My daughter does it too. What we do so far is ask her to spit it out. It seems to work but i know it's not ideal. I am hoping if we keep doing that, she knows it's not allowed..

Thanks Everyone I am trying to look into Autism swim lessons in hopes of helping with this problem. It makes it more challenging since he is mostly non verbal right now

Thanks. Glad to know I am not alone

You may be able to sign "water yuckie" it will be hard work !! But it gives him a visual with the words you say and he may catch on better.
But I have the same problem as well.